In the class of hearing aid systems generally referred to as implantable hearing instruments, some or all of various hearing augmentation componentry is positioned subcutaneously on or within a patient's skull, typically at locations proximate the mastoid process. In this regard, implantable hearing instruments may be generally divided into two sub-classes, namely semi-implantable and fully implantable. In a semi-implantable hearing instrument, one or more components such as a microphone, signal processor, and transmitter may be externally located to receive, process, and inductively transmit an audio signal to implanted components such as a transducer. In a fully implantable hearing instrument, typically all of the components, e.g., the microphone, signal processor, and transducer, are located subcutaneously. In either arrangement, an implantable transducer is utilized to stimulate a component of the patient's auditory system (e.g., ossicles and/or the cochlea).
By way of example, one type of implantable transducer includes an electromechanical transducer having a magnetic coil that drives a vibratory actuator. The actuator is positioned to interface with and stimulate the ossicular chain of the patient via physical engagement. (See e.g., U.S. Pat. No. 5,702,342). Another implantable transducer utilizes implanted exciter coils to electromagnetically stimulate magnets affixed in the middle ear (See e.g., U.S. Pat. No. 5,897,486). In both arrangements, one or more bones of the ossicular chain are made to mechanically vibrate, which causes the ossicular chain to stimulate the cochlea through its natural input, the so-called oval window. For purposes hereof, electromechanical transducers capable of stimulating auditory components within the tympanic cavity, including the tympanic membrane, the ossicular chain and/or the oval window, are collectively referred to as “middle ear transducers.”
Movement of the ossicular chain results in the displacement of fluid within the cochlea, which in turn results in the sensation of sound. The displacement of fluid is caused by the interaction of the innermost ossicle bone, the stapes, with the oval window, wherein the stapes functions similar to a piston moving against fluid within/behind the oval window. In a healthy ear, vibrations of the tympanic membrane cause natural movement of the ossicular chain (e.g., through the malleaus, incus and stapes). This natural movement causes the stapes to move in an up-and-down manner that is substantially normal to the interface between the stapes and the oval window. This natural movement typically provides the most effective transfer of energy to the oval window. That is, the greatest hearing ‘gain’ is achieved by natural movement of the stapes, which is associated with natural movement of the malleus and incus. One difficulty that arises in stimulating a middle ear component with a middle ear transducer is achieving natural movement of one or more bones of the ossicular chain.
Often, a middle ear transducer mechanically vibrates the ossicular chain (i.e., in response to a transducer stimulation signal) in a non-natural direction. As may be appreciated, the utilization of an implantable middle ear transducer generally entails surgical positioning of the transducer. Such positioning may be within the mastoid process of a patient's skull and require the insertion of an elongated vibratory actuator through a hole drilled in the mastoid process. The elongated vibratory actuator may then extend into the tympanic cavity. Due to the position of the ear canal, the hole drilled through the mastoid process generally intersects the tympanic cavity in a region of the cavity where the incus and malleus are found. In this case, the elongated vibratory actuator may be coupled to one of these ossicle bones during mounting and positioning of the transducer within the patient. In one example, such coupling may occur via a small aperture formed in the incus bone.
Typically, such an elongated vibratory actuator is driven along its length to provide axial vibrations. In many instances, this ‘axial’ direction of movement is not aligned in the direction of natural movement of the incus and malleus. While this axial movement of the ossicular chain results in stimulation of the cochlea and the sensation of sound, the applied stimulation signal is not optimally transferred. Accordingly, enhanced hearing gain could be realized by providing more natural movement of the ossicular chain.